[en] In teleost fishes, the lateral system is assumed to contribute, among other roles, to maintaining schooling behaviour. Sight is also assumed to play a role in schooling, as fish with a cut lateral line do not stop schooling unless they are also blinded. This conclusion, however, is based on experiments where only the trunk lateral line was inactivated, leaving the head lateral system intact. We investigated how inactivation of the whole lateral system affects fish shoaling behaviour. Groups of firehead tetras, Hemigrammus bleheri, were videorecorded before and after inactivation of their whole lateral system with aminoglycoside antibiotics (and also in sham-treated specimens). Shoaling behaviour was characterized by nearest distance to the first, second and third neighbours, shoal radius, shoal order parameter and the number of collisions between individuals. Scanning electron microscope observations showed damage to most superficial neuromasts as a result of antibiotic treatment. Importantly, the antibiotic-treated fish proved unable to maintain a shoal. After the end of the treatment, however, they recovered both a normal tissue morphology and normal shoaling behaviour within about a month. The lateral system is thus more crucial to shoaling behaviour than previously believed.